Nuclear power generation uses heat generated by a nuclear fission reaction of nuclear fuel in a reactor pressure vessel. The heat generated by the nuclear fission reaction is transferred to a primary side circulated between a fuel assembly and a steam generator. Heated water of the primary side transfers heat from the steam generator to a secondary side, and steam generated as a result of the heat transfer operates a turbine generator. After a temperature of the secondary side passing through the turbine is decreased in a condenser, the secondary side water again enters the steam generator.
In this case, cooling water flowing in the primary side, that is, water circulated between a nuclear reactor and the steam generator is called primary cooling water, and cooling water flowing in the secondary side is called secondary cooling water.
A tube sheet 100 included in the steam generator operated in a nuclear power plant is a support plate expanding and fixing a tube 200 and is made of a low-alloy steel material.
The primary cooling water flows inside the tube 200 and the secondary cooling water flows outside the tube 200.
Referring to FIG. 1, a surface of a secondary tube sheet in contact with the secondary cooling water is oxidized to magnetite by a corrosion reaction, such that a volume thereof is expanded. In this process, an outer surface of the tube 200 adjacent to the oxidized surface is pressed in an inner diameter direction, such that a shape thereof is changed. This phenomenon is called denting. When the denting is generated, stress is applied to the tube 200, such that a stress corrosion crack of the tube 200 is easily generated.
In the case in which the crack is generated in the tube 200, the primary cooling water flowing inside the tube 200 is leaked to the outside of the tube 200. The primary cooling water includes metal ions or metal oxides such as cobalt, nickel, iron, or the like, dissolved from a surface of a material that it contacts. These metal ions and metal oxides are activated in the nuclear reactor core. Therefore, the activated primary cooling water is leaked to the outside of the tube 200 to thereby be discharged to the outside of the steam generator together with the secondary cooling water.
As an existing method for suppressing denting, a water chemistry control method of maintaining quality of secondary cooling water to be neutral and a lancing and chemical cleaning method of removing sludge 40 accumulated on the tube sheet have been applied. However, the denting phenomenon has been continuously generated.
In addition, Korean Patent Laid-Open Publication No. 2005-0007950, which relates to a method of preventing corrosion of a tube sheet 100 of a steam generator, has disclosed a configuration of reinforcement-welding the tube sheet 100 using a strip made of a strong anticorrosive material. However, the reinforcement-welding is performed after forming a tube mounting hole 30, and the tube mounting hole 30 is again formed after cladding-welding. In addition, the reinforcement-welding is performed up to an inner portion of the tube sheet 100.